i think it may be 4n... here's whythe tetraploid plant's pollen grain would then be diploid, no?so 2n combines with the polar nuclei... 2n+n+n= 4nnormally endosperm are triploid, and i thought they always were... but apparently that's not always the case. if you haven't, look herehttp://en.wikipedia.org/wiki/Endosperm